remove uses of golang.org/x/sys/execabs

the "golang.org/x/sys/execabs" package was introduced to address a security
issue on Windows, and changing the default behavior of os/exec was considered
a breaking change. go1.19 applied the behavior that was previously implemented
in the execabs package;

from the release notes: https://go.dev/doc/go1.19#os-exec-path

> Command and LookPath no longer allow results from a PATH search to be found
> relative to the current directory. This removes a common source of security
> problems but may also break existing programs that depend on using, say,
> exec.Command("prog") to run a binary named prog (or, on Windows, prog.exe)
> in the current directory. See the os/exec package documentation for information
> about how best to update such programs.
>
> On Windows, Command and LookPath now respect the NoDefaultCurrentDirectoryInExePath
> environment variable, making it possible to disable the default implicit search
> of “.” in PATH lookups on Windows systems.

With those changes, we no longer need to use the execabs package, and we can
switch back to os/exec.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

go.mod

@@ -1,8 +1,7 @@
 module github.com/docker/docker-credential-helpers
 
-go 1.18
+go 1.19
 
-require (
-	github.com/danieljoos/wincred v1.1.2
-	golang.org/x/sys v0.7.0
-)
+require github.com/danieljoos/wincred v1.1.2
+
+require golang.org/x/sys v0.7.0 // indirect